Astronomical Units Research Articles

Water UV-shielding in the Terrestrial Planet-forming Zone: Implications from Water Emission

Mid-infrared spectroscopy is one of the few ways to observe the composition of the terrestrial planet-forming zone, the inner few astronomical units, of protoplanetary disks. The species currently detected in the disk atmosphere, for example, CO, CO2, H2O, and C2H2, are theoretically enough to constrain the C/O ratio on the disk surface. However, thermochemical models have difficulties in reproducing the full array of detected species in the mid-infrared simultaneously. In an effort to get closer to the observed spectra, we have included water UV-shielding as well as more efficient chemical heating into the thermochemical code Dust and Lines. We find that both are required to match the observed emission spectrum. Efficient chemical heating, in addition to traditional heating from UV photons, is necessary to elevate the temperature of the water-emitting layer to match the observed excitation temperature of water. We find that water UV-shielding stops UV photons from reaching deep into the disk, cooling down the lower layers with a higher column. These two effects create a hot emitting layer of water with a column of 1–10 × 1018 cm−2. This is only 1%–10% of the water column above the dust τ = 1 surface at mid-infrared wavelengths in the models and represents <1% of the total water column.

Observing Planet-driven Dust Spirals with ALMA

Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations of thermal emission from the dust component of protoplanetary disks have revealed an abundance of substructures that may be interpreted as evidence for embedded planets, but planet-driven spiral arms—perhaps one of the most compelling lines of evidence—have proven comparatively elusive. In this work, we test the capabilities of ALMA to detect the planet-driven spiral signal in continuum emission. Carrying out hydrodynamic simulations and radiative transfer calculations, we present synthetic Band 7 continuum images for a wide range of disk and observing conditions. We show that thermal mass planets at tens of astronomical units typically drive spirals detectable within a few hours of integration time, and the detectable planet mass may be as low as ∼Neptune mass (0.3 M th). The grains probed by ALMA form spirals morphologically identical to the underlying gas spiral. The temperature of the dust spiral is crucial in determining its contrast, and spirals are easier to detect in disks with an adiabatic equation of state and longer cooling times. Resolving the spiral is not necessary for its detection; with the help of residual maps, the optimal beam size is a few times the spiral width at a constant noise level. Finally, we show how the presence of gaps and rings can impair our ability to recognize colocated spirals. Our work demonstrates the planet-finding potential of the current design specification of ALMA, and suggests that observing capability is not the bottleneck in searching for spirals induced by thermal mass planets.

Natural separation of two primordial planetary reservoirs in an expanding solar protoplanetary disk.

Meteorites display an isotopic composition dichotomy between noncarbonaceous (NC) and carbonaceous (CC) groups, indicating that planetesimal formation in the solar protoplanetary disk occurred in two distinct reservoirs. The prevailing view is that a rapidly formed Jupiter acted as a barrier between these reservoirs. We show a fundamental inconsistency in this model: If Jupiter is an efficient blocker of drifting pebbles, then the interior NC reservoir is depleted by radial drift within a few hundred thousand years. If Jupiter lets material pass it, then the two reservoirs will be mixed. Instead, we demonstrate that the arrival of the CC pebbles in the inner disk is delayed for several million years by the viscous expansion of the protoplanetary disk. Our results support the hypothesis that Jupiter formed in the outer disk (>10 astronomical units) and allowed a considerable amount of CC material to pass it and become accreted by the terrestrial planets.

Von Karman correlation similarity in solar wind magnetohydrodynamic turbulence.

A major development underlying hydrodynamic turbulence theory is the similarity decay hypothesis due to von Karman and Howarth, here extended empirically to plasma turbulence in the solar wind. In similarity decay the second-order correlation experiences a continuous transformation based on a universal functional form and a rescaling of energy and characteristic length. Solar wind turbulence follows many principles adapted from classical fluid turbulence, but previously this similarity property has not been examined explicitly. Here, we analyze an ensemble of Elsässer autocorrelation functions computed from Advanced Composition Explorer data at 1 astronomical unit (AU), and demonstrate explicitly that the two-point correlation functions undergo a collapse to a similarity form of the type anticipated from von Karman's hypothesis applied to weakly compressive magnetohydrodynamic turbulence. This provides a firm empirical basis for employing the similarity decay hypothesis to the Elsässer correlations that represent the incompressive turbulence cascade. This approach is of substantial utility in space turbulence data analysis, and for adopting von Karman-type heating rates in global and subgrid-scale dynamical modeling.

Comet fading begins beyond Saturn.

The discovery probability of long-period comets (LPCs) passing near the Sun is highest during their first passage and then declines, or fades, during subsequent return passages. Comet fading is largely attributed to devolatilization and fragmentation via thermal processing within 2 to 3 astronomical unit (au) of the Sun (1 au being the Earth-Sun distance). Here, our numerical simulations show that comet-observing campaigns miss vast numbers of LPCs making returning passages through the Saturn region (near 10 au) because these comets fade during prior, even more distant passages exterior to Saturn and thus elude detection. Consequently, comet properties substantially evolve at solar distances much larger than previously considered, and this offers new insights into the physical and dynamical properties of LPCs, both near and far from Earth.

Voyager's observations in the vicinity of the heliopause

This work analyzes Voyager 2 observations on November 2018 and compares them with Voyager 1 data at the vicinity of the heliopause in July-August 2012. We describe the plasma and cosmic-ray variations at the radial distance of $\approx$ 1 astronomical unit (AU) from the heliopause. We use a simple convection-diffusion cosmic-ray modulation model to qualitatively explain the particle observations. We found a thin layer, with a thickness of $\approx$ 0.04 AU where the radial component of the solar wind speed vanished, the galactic cosmic ray intensity rapidly increased to reach its heliosphere boundary level, and low-energy heliospheric ion intensity drooped. We called this layer the "skin of the heliosphere". Plasma data suggest that Voyager 2 crossed the heliopause on November 5, 2018, at the radial distance of 119.03 AU. We apply our analysis to Voyager 1 observations and conclude that similar behavior in solar wind speed could qualitatively explain the GCR counting rate and that the "skin of the heliosphere" maybe a global characteristic along the heliopause.

Birth cluster simulations of planetary systems with multiple super-Earths: initial conditions for white dwarf pollution drivers

ABSTRACT Previous investigations have revealed that eccentric super-Earths represent a class of planets that are particularly effective at transporting minor bodies towards white dwarfs and subsequently polluting their atmospheres with observable chemical signatures. However, the lack of discoveries of these planets beyond a few astronomical units from their host stars prompts a better understanding of their orbital architectures from their nascent birth cluster. Here, we perform stellar cluster simulations of three-planet and seven-planet systems containing super-Earths on initially circular, coplanar orbits. We adopt the typical stellar masses of main-sequence progenitors of white dwarfs ($1.5\, \mathrm{M}_{\odot }$–$2.5\, \mathrm{M}_{\odot }$) as host stars and include 8000 main-sequence stars following a Kroupa initial mass function in our clusters. Our results reveal that about 30 per cent of the simulated planets generate eccentricities of at least 0.1 by the time of cluster dissolution, which would aid white dwarf pollution. We provide our output parameters to the community for potential use as initial conditions for subsequent evolution simulations.

Dust Dynamics in Transitional Disks: Clumping and Disk Recession

Abstract The role of radiation pressure in dust migration and the opening of inner cavities in transitional disks is revisited in this paper. Dust dynamics including radiation pressure is often studied in axisymmetric models, but in this work, we show that highly non-axisymmetric features can arise from an instability at the inner disk edge. Dust grains clump into high density features there, allowing radiation to leak around them and penetrate deeper into the disk, changing the course of dust migration. Our proof-of-concept, two-dimensional, vertically averaged simulations show that the combination of radiation pressure, shadowing, and gas drag can produce a net outward migration, or recession, of the dust component of the disk. The recession speed of the inner disk edge is on the order of 10−5 times Keplerian speed in our parameter space, which is faster than the background viscous flow, assuming a Shakura–Sunyaev viscosity α ≲ 10−3. This speed, if sustained over the lifetime of the disk, can result in a dust cavity as large as tens of astronomical units.

Voyager 3: A Concept Mission to Interstellar Medium

Voyager 3 is a concept mission that sends a space telescope to the interstellar medium in a reasonable amount of time. Voyager 3 would take a direct image of an exoplanet using the solar gravitational lensing at the distance of 550 astronomical unit (AU). The spacecraft would use its suite of scientific instruments to study the environment of the local solar system and interstellar medium and finalize its primary mission by imaging an exoplanet. Two potential architectures are proposed to meet this mission directive, using multiple gravitational assists and sizable electric propulsion burns to achieve the high escape speeds necessary to reach 550 AU. This paper expands on the science mission objectives, trajectory, and the preliminary design of the baseline spacecraft.

Navigating stellar wobbles for imaging with the solar gravitational lens

The solar gravitational lens (SGL) offers unique capabilities for direct high-resolution imaging of faint, distant objects, such as exoplanets. For that purpose, in the near future, a spacecraft carrying a meter-class telescope with a solar coronagraph would be placed in the focal region of the SGL. That region begins at $\ensuremath{\sim}547$ astronomical units from the Sun and occupies the immediate vicinity of the target-specific primary optical axis---the line that connects the center of the target and that of the Sun. Clearly, this axis is not at rest. It undergoes complex motion as the exoplanet orbits its host star, as that star moves with respect to the Sun, and even as the Sun itself moves with respect to the Solar System's barycenter due to the gravitational pull of planets in our Solar System. Although less prominent, other motions exist. An image of an extended object is projected by the SGL into an image plane and moves within that plane, responding to the motion of the optical axis. To sample the image, a telescope must always be on the move, following the projection, with precise knowledge of its own position with respect to the image. We consider the dominant motions that determine the position of the focal line as a function of time. We evaluate the needed navigational capability for the telescope to conduct a multiyear exoplanet imaging mission in the focal region for the SGL. We show that even in a rather conservative case, when an Earth-like exoplanet is in our immediate stellar neighborhood at $\ensuremath{\sim}10$ light years, the motion of the image is characterized by a small total acceleration that is driven primarily by the orbital motion of the exoplanet (its effect on the projected image estimated to be at the level of $\ensuremath{\sim}6\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}/\mathrm{s}}^{2}$, decreasing inversely with distance to a target) and by the reflex motion of our Sun (target independent, contributing at $\ensuremath{\sim}0.2\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}/\mathrm{s}}^{2}$). We discuss how the amplified light of the host star allows us to establish a local reference frame that significantly relaxes navigational requirements for the imaging operations. We conclude that the required navigation in the SGL's focal region, although complex, can be accurately modeled, and a $\ensuremath{\sim}10$-year prospective imaging mission is achievable with the already available propulsion technology.

Enrichment of Jupiter’s Atmosphere by Late Planetesimal Bombardment

Abstract Jupiter’s atmosphere is enriched with heavy elements by a factor of about 3 compared to a protosolar composition. The origin of this enrichment and whether it represents the bulk composition of the planetary envelope remain unknown. Internal structure models of Jupiter suggest that its envelope is separated from the deep interior and that the planet is not fully mixed. This implies that Jupiter’s atmosphere was enriched with heavy elements just before the end of its formation. Such enrichment can be a result of late planetesimal accretion. However, in situ Jupiter formation models suggest a decreasing accretion rate with increasing planetary mass, which cannot explain Jupiter’s atmospheric enrichment. In this study, we model Jupiter’s formation and show that the migration of proto-Jupiter from ∼20 au to its current location can lead to late planetesimal accretion and atmospheric enrichment. Late planetesimal accretion does not occur if proto-Jupiter migrates only a few astronomical units. We suggest that if Jupiter’s outermost layer is fully mixed and is relatively thin (up to ∼20% of its mass), such late accretion can explain its measured atmospheric composition. It is therefore possible that Jupiter underwent significant orbital migration followed by late planetesimal accretion.

The Monte San Nicola section (Sicily) revisited: A potential unit-stratotype of the Gelasian Stage

The Monte San Nicola area (Southern Sicily) offers a spectacular exposure of open-marine sediments that were employed in 1998 for defining the Global Stratotype Section and Point (GSSP) of the Gelasian Stage (Upper Pliocene). After the lowering of the Pliocene/Pleistocene boundary to ca. 2.6 Ma in 2010, the Gelasian GSSP has been redefined as the base of both the Pleistocene Series and the Quaternary Period, which increased its importance and visibility within the scientific community. However, documentation on the Monte San Nicola reference section is still sparse. In the light of its renewed status, we decided to undertake a complete revision of the Gelasian Stage in its type area, in order to evaluate whether the succession of bio- and magnetostratigraphic events that are expected to occur in the interval of relevance are represented adequately in the local record. The results of our investigation demonstrate that the Monte San Nicola succession spans continuously from the upper Piacenzian to the lower Calabrian, and is therefore suitable to host the Unit Stratotype, or even the Astronomical Unit Stratotype, of the Gelasian Stage.

Пространство и время в мифоисторической памяти манси

The paper is devoted to the reconstruction in historical retrospection of the images of the space-time continuum, expressed through the material world of the Mansis ethnic culture and concentrated in the folklore sphere. The methodology is set by the postulates about the specificity of mythological thinking, refracted in relation to time and space. The source base was made up of the Mansis folklore texts collected from the beginning of the 20th century. Following his predecessors, the author distinguishes three periods in the mythological and poetic history of the Mansis. The classification is based on the principle of generational affiliation of the main characters. The first period is associated with the era of primordial creation and the “old couple” or the supreme deity Num-Torum. Time and space in it are substantial and are expressed through the images of water and earth. Infinity, as the initial state of space, is replaced by a measurement through time, which emphasizes its finiteness and limitation. The principle of trinity dominates in the reckoning of time. The second period is set by the acts of the sons of the supreme deity - Taryg-peshch-nimal'-sova and the bear. The space is desribed multidimensional: the earth and near-earth spheres -the sky, the underground, the water and the southern land of Mortim-maa, as well as the underworld or the dead world. Particular attention is paid to the “bark soil”, which is characterized by landscape: first of all, rivers, but also swamps, thickets, mountains, manes. The idea of “one's own land” as a personal form of connection with space is fixed. The culture of the space is emphasized, the signs of which are villages, fortresses, barns, plague, and the sacred part - “sacred place” - stands out. The perception of time is also distinguished by multidimensionality. On the one hand, relativity is attributed to it due to the opposition “far -close”, correlated with movement, on the other hand, there is time reckoning with the help of the light and dark parts of the day, accentuation of morning and evening as the prologue and epilogue of the day. The third period becomes the arena of human action, and in various folklore forms of manifestation of human nature - Ekva-pyris, people-heroes, the progenitors of the phratries Por and Mos. The main thing here is activities and actions through which the feeling of timespace is transmitted. Space is thought of locally, although the degree of local concretization is different: on the one hand, an indefinitely distant territory, and on the other, a topographically verified reference to real settlements and geographical objects. The interpretation of time, and above all in the heroic epic, is as close as possible to the linear historical. During this period, there is the most developed system of calculating time, in which astronomical units dominate - the year as the sum of winter and summer, day and night. At the same time, the day is interpreted both as the time of human life and life itself. The presence of spatio-temporal portals intersperses the specifics of the expression of space and time, inherent in periods, in the general picture of the Mansis world.

Point-spread Function Deconvolution of the IFU Data and Restoration of Galaxy Stellar Kinematics

Abstract We present a performance test of the point-spread function (PSF) deconvolution algorithm applied to astronomical integral field unit (IFU) spectroscopy data for restoration of galaxy kinematics. We deconvolve the IFU data by applying the Lucy–Richardson algorithm to the 2D image slice at each wavelength. We demonstrate that the algorithm can effectively recover the true stellar kinematics of the galaxy, by using mock IFU data with a diverse combination of surface brightness profile, signal-to-noise ratio, line-of-sight geometry, and line-of-sight velocity distribution (LOSVD). In addition, we show that the proxy of the spin parameter λ R e can be accurately measured from the deconvolved IFU data. We apply the deconvolution algorithm to the actual SDSS-IV MaNGA IFU survey data. The 2D LOSVD, geometry, and λ R e measured from the deconvolved MaNGA IFU data exhibit noticeable differences compared to the ones measured from the original IFU data. The method can be applied to any other regular-grid IFU data to extract the PSF-deconvolved spatial information.

Formation of the Earth and Moon: Influence of Small Bodies

The paper discusses a model of the bombardment of the Earth and the Moon by small bodies when these planets were formed. It is shown that the total ice mass delivered with the bodies to the Earth from the feeding zone of the giant planets and the outer asteroid belt could have been comparable to the total mass of the Earth’s oceans. Objects that initially crossed Jupiter’s orbit could become Earth-crossers mainly within the first one million years. Most collisions of bodies originally located at a distance of 4 to 5 AU (astronomical units) from the Sun with the Earth occurred during the first ten million years. Some bodies from the Uranus and Neptune zones could fall onto the Earth in more than 20 million years. From their initial distances from the Sun of about 3 to 3.5 AU, some bodies could fall onto the Earth and Moon in a few billion years for the model that takes into account only the gravitational influence of the planets. The ratio of the number of bodies that collided with the Earth to the number of bodies that collided with the Moon varied mainly from 20 to 40 for planetesimals from the feeding zone of the terrestrial planets. For bodies originally located at a distance of more than 3 AU from the Sun, this ratio was mainly in the range between 16.4 and 17.4. The characteristic velocities of collisions of planetesimals from the feeding zones of the terrestrial planets with the Moon varied from 8 to 16 km/s, depending on the initial values of the semi-major axes and eccentricities of orbits of the planetesimals. The collision velocities of bodies that came from the feeding zones of Jupiter and Saturn with the Moon were mainly from 20 to 23 km/s.

Cooling-induced Vortex Decay in Keplerian Disks

Abstract Vortices are readily produced by hydrodynamical instabilities, such as the Rossby wave instability, in protoplanetary disks. However, large-scale asymmetries indicative of dust-trapping vortices are uncommon in submillimeter continuum observations. One possible explanation is that vortices have short lifetimes. In this paper, we explore how radiative cooling can lead to vortex decay. Elliptical vortices in Keplerian disks go through adiabatic heating and cooling cycles. Radiative cooling modifies these cycles and generates baroclinicity that changes the potential vorticity of the vortex. We show that the net effect is typically a spin down, or decay, of the vortex for a subadiabatic radial stratification. We perform a series of two-dimensional shearing box simulations, varying the gas cooling (or relaxation) time, t cool, and initial vortex strength. We measure the vortex decay half-life, t half, and find that it can be roughly predicted by the timescale ratio t cool/t turn, where t turn is the vortex turnaround time. Decay is slow in both the isothermal (t cool ≪ t turn) and adiabatic (t cool ≫ t turn) limits; it is fastest when t cool ∼ 0.1 t turn, where t half is as short as ∼300 orbits. At tens of astronomical units where disk rings are typically found, t turn is likely much longer than t cool, potentially placing vortices in the fast decay regime.

V488 Per Revisited: No Strong Mid-infrared Emission Features and No Evidence for Stellar/substellar Companions

Abstract We present characterization of the planetary system architecture for V488 Per, the dustiest main-sequence star known with a fractional infrared luminosity of ≈16%. Far-infrared imaging photometry confirms the existence of an outer planetary system dust population with a blackbody-fit temperature of ≈130 K. Mid-infrared spectroscopy probing the previously identified ≈800 K inner planetary system dust population does not detect any obvious solid-state emission features, suggesting either large grain sizes that mute such emission and/or grain compositions dominated by species like amorphous carbon and metallic iron, which do not produce such features. In the latter case, the presence of significant quantities of iron-rich material could be indicative of the active formation of a Mercury-like planet around V488 Per. In any event, the absence of solid-state emission features is very unusual among main-sequence stars with copious amounts of warm orbiting dust particles; we know of no other such star whose mid-infrared spectrum lacks such features. Combined radial velocity monitoring and adaptive optics imaging find no evidence for stellar/substellar companions within several hundred astronomical units of V488 Per.

Molecules with ALMA at Planet-forming Scales (MAPS). XVI. Characterizing the Impact of the Molecular Wind on the Evolution of the HD 163296 System

Abstract During the main phase of evolution of a protoplanetary disk, accretion regulates the inner-disk properties, such as the temperature and mass distribution, and in turn, the physical conditions associated with planet formation. The driving mechanism behind accretion remains uncertain; however, one promising mechanism is the removal of a fraction of angular momentum via a magnetohydrodynamic (MHD) disk wind launched from the inner tens of astronomical units of the disk. This paper utilizes CO isotopologue emission to study the unique molecular outflow originating from the HD 163296 protoplanetary disk obtained with the Atacama Large Millimeter/submillimeter Array. HD 163296 is one of the most well-studied Class II disks and is proposed to host multiple gas-giant planets. We robustly detect the large-scale rotating outflow in the 12CO J = 2 − 1 and the 13CO J = 2 − 1 and J = 1 − 0 transitions. We constrain the kinematics, the excitation temperature of the molecular gas, and the mass-loss rate. The high ratio of the rates of ejection to accretion (5–50), together with the rotation signatures of the flow, provides solid evidence for an MHD disk wind. We find that the angular momentum removal by the wind is sufficient to drive accretion though the inner region of the disk; therefore, accretion driven by turbulent viscosity is not required to explain HD 163296's accretion. The low temperature of the molecular wind and its overall kinematics suggest that the MHD disk wind could be perturbed and shocked by the previously observed high-velocity atomic jet. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Boson dark matter halos with a dominant noncondensed component

We consider galaxy halos formed by dark matter bosons with mass in the range of about a few tens or hundreds eV. A major part of the particles is in a noncondensed state and described under the Thomas-Fermi approach. Derived equations are solved numerically to find the halo density profile.The noncondensed state is supported in the entire halo except compact gravitationally bounded Bose-Einstein condensates. Although the size of these compact objects, also known as Bose stars, depends on interactions between the particles, its upper limit is only about 100 astronomical units. The Bose stars collect the condensed bosons providing a density cusp avoidance in the halo as well as a natural mechanism to prevent overproduction of small halos.Clusters of the Bose stars can also contribute to the halo density profile.The model is analyzed by confronting its predictions with observations of galaxy rotation curves. We employ 22 low surface brightness galaxies and obtain that the model is consistent with the observational data when the particle mass is in the range above about 50 eV and the best fit corresponds to the mass m = 86 eV. This mass is appropriate for relic dark matter bosons, which decouple just after QCD phase transition.

A Jovian analogue orbiting a white dwarf star.

Studies1,2 have shown that the remnants of destroyed planets and debris-disk planetesimals can survive the volatile evolution of their host stars into white dwarfs3,4, but few intact planetary bodies around white dwarfs have been detected5-8. Simulations predict9-11 that planets in Jupiter-like orbits around stars of ≲8 M☉ (solar mass)avoid being destroyed by the strong tidal forces of their stellar host, but as yet, there has been no observational confirmation of such a survivor. Here we report the non-detection of a main-sequence lens star in the microlensing event MOA-2010-BLG-477Lb12 using near-infrared observations from the Keck Observatory. We determine that this system contains a 0.53 ± 0.11 M☉ white-dwarf host orbited by a 1.4±0.3 Jupiter-mass planet with a separation on the plane of the sky of 2.8 ± 0.5 astronomical units, which implies a semi-major axis larger than this. This system is evidence that planets around white dwarfs can survive the giant and asymptotic giant phases of their host's evolution, and supports the prediction that more than half of white dwarfs have Jovian planetary companions13. Located at approximately 2.0 kiloparsecs towards the centre of our Galaxy, it is likely torepresent an analogue to the end stages of the Sun and Jupiter in our own Solar System.